The field of the invention is multi-modality imaging systems and methods, such as positron emission tomography (PET) and magnetic resonance imaging systems and methods. More particularly, the present invention relates to improved time-of-flight PET/MR imaging systems and methods.
Positrons are positively charged electrons which are emitted by radionuclides that have been prepared using a cyclotron or other device. These are employed as radioactive tracers called “radiopharmaceuticals” by incorporating them into substances, such as glucose or carbon dioxide. The radiopharmaceuticals are administered to a patient and become involved in biochemical or physiological processes such as blood flow; fatty acid and glucose metabolism; and protein synthesis.
As the radionuclides decay, they emit positrons. The positrons travel a very short distance before they encounter an electron, and when this occurs, they are annihilated and converted into two photons, or gamma rays. This annihilation event is characterized by two features which are pertinent to PET scanners—each gamma ray has an energy of 511 keV and the two gamma rays are directed in nearly opposite directions. An image indicative of the tissue concentration of the positron emitting radionuclide is created by determining the number of such annihilation events at each location within the field of view.
The PET scanner includes one or more rings of detectors which encircle the patient and which convert the energy of each 511 keV photon into a flash of light that is sensed by a photomultiplier tube (PMT). Coincidence detection circuits connect to the detectors and record only those photons which are detected simultaneously by two detectors located on opposite sides of the patient. The number of such simultaneous events indicates the number of positron annihilations that occurred along a line joining the two opposing detectors. Within a few minutes hundreds of million of events are recorded to indicate the number of annihilations along lines joining pairs of detectors in the ring. These numbers are employed to reconstruct an image using well known computed tomography techniques.
Positron emission tomography provides quantitative images depicting the concentration of the positron emitting substance throughout the patient. The accuracy of this quantitative measurement depends in part on the accuracy of an attenuation correction that accounts for the absorption of some of the gamma rays as they pass through the patient. The attenuation correction factors modify the sinogram which contains the number of annihilation events at each location within the field of view. There are a number of methods used to measure, or calculate the attenuation factors. These include calculating the attenuation correction; measuring attenuation correction; and a hybrid, or segmented tissue technique. In this regard, attenuation correction is essential for the quantitation of PET data. However, as described above, this can be difficult to achieve with a PET system. There are several methods proposed to estimate attenuation correction from emission PET data; however, each has shortcomings.
As such, it has been recently proposed by M. Defrise, A. Rezaei, and J. Nuyts, “Time-of-flight PET data determine the attenuation sinogram up to a constant,” Phys. Med. Biol., vol. 57, no. 4, pp. 885, 2012, that an attenuation sinogram that can be uniquely reconstructed from ideal time-of-flight (TOF) PET emission data by estimating the gradient of an attenuation sinogram using a TOF PET data consistency condition. Defrise et al. propose a two-step method to compute attenuation correction; however, the method suffers from unstable estimations of high frequency components and also areas with low photo counts, even without the presence of Poisson noise.
Accordingly, there is a need for a robust system and method for determining attenuation correction in PET imaging.